CD4 T helper cell differentiation is controlled by 'master'transcription factors that commit the cell to become a Th1, Th2 or Thi 7 cell or a Treg. It was previously believed that the expression of these transcription factors was mutually exclusive. However, recently, it has been demonstrated that T helper cells can express multiple 'master'transcription factors in vivo. How these individual transcription factors induce their specific transcription profile is well understood, yet little is known about how these 'master'transcription factors interact in cells co-expressing them. Based on our preliminary data on cells expressing both Foxp3 and T- bet, the transcriptome of these T-bet-expressing Tregs is unique. While some Thi-specific genes are expressed (e.g., CXCR3), the expression of other Thi-associated genes is suppressed (e.g., IFNy). Despite the fact that a direct role of Foxp3 in the selective suppression of the T-bet transcriptome has not been shown, and, given the important role of Foxp3 in suppressing gene transcription in Tregs, it is tempting to speculate that Foxp3 plays a critical role in the differential silencing of Thi genes in these cells. The experiments proposed in this application aim to determine the role of Foxp3 in this repression and the mechanism by which FoxpS mediates the silencing. Cell lines that stably and inducibly express Foxp3, will be generated and transfected with T-bet-expressing plasmids. Using high density microarrays, the transcriptional profile of these cells will be analyzed to identify a panel of Thi-specific genes downregulated in Foxp3-expressing cells. We will then examine the epigenetic modifications induced by Foxp3 at these promoters, comparing these changes to Thi-specific genes that are not affected by Foxp3 expression. With the tools generated in this proposal we will gain an unprecedented understanding of not only the regulatory mechanism utilized by FoxpS in silencing gene transcription, but also the importance of histone modifications in driving T cell development and differentiation. Furthermore, a detailed understanding of how FoxpS functions to silence pro-inflammatory genes, like IFNy, may allow for the development of therapies to limit inflammation in autoimmune diseases such as type 1 diabetes, rheumatoid arthritis and multiple sclerosis, or boost the immune response by dampening Treg control in chronic infections, vaccination and cancer PUBLIC HEALTH RELEVANCE: understanding how the expression of pro-inflammatory proteins is inhibited may allow for the development of novel anti-inflammatory therapies. As, many autoimmune disease are caused by excessive inflammation, this has implicated for a number of conditions including type I diabetes, rheumatoid arthritis and multiple sclerosis. Alternatively, treatments preventing this inhibition may be used to boost the immune response in chronic viral infections, vaccination or cancer.